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H58                         Appendix VI.

has the cylinder been water-jacketed, but water has been supplied
to the pistons through jointed or telescopic pipes, flow and return,
I                              and even to the interior of the exhaust valve.

j    !                               Scavenging was introduced at an early period in the Griffin

and Beck engines (see p. 912), but the loss of two strokes per
,                               cycle was a serious matter, and the advantages of scavenging were

\\ *                           therefore considered doubtful.    Semi-scavenging was performed

jj   ,                          in Clerk's engine, p. 911, where the charge, pupared beforehand,

,* [/                          was suddenly admitted, and expelled the exhaust through ample

'j                              ports.    The introduction of large engines with poor gas compelled

j ,.                          the re-introduction of scavenging, when it was discovered that

I i   "                          the effect could be obtained in two different ways, now known as

positive and non-positive. The first of these has been already
suggested, and the second, first used by Mr. Atkinson, consists in
providing a large exhaust valve kept open longer than usual, and
expelling the products into a pipe of about 65 feet length, when
the wave action of the gases was such as to cause a partial vacuum
of 2 Ibs. below the atmosphere, and draw the whole contents
away from the cylinder, while permitting the entrance of air
thereto through an automatically-opening air valve. This second
method, though suitable for the smaller engines, gives way to the
positive method with auxiliary air pump in the larger engines.
In any case, a power improvement of about 25% is obtained by
the clearing process.

Greater compression has been possible with the before-
mentioned improvements, without which mean pressures above
60 Ibs. caused piston and valve overheating, and an invitation to
seizing. Mean pressures have been gradually increased, first to
85, then to iio and even more Ibs. per sq. in.; in fact, the main
improvement in thermal efficiency may be ascribed to increased
compression, whose effect is shewn clearly in the following results?
of the Gas Engine Research Committee :

/Compression pressure in Ibs,                 Efficiency on B.H.P.

''.," .-   ../55   -'         -v :' ... . 13*2,
71  ...     ...     ..'..   ....' 14-9

>.. . ,    ,  ,,    93    .-   '    .-        ...       r-    13*4
,/     '.           124    ...        ... '     ...        ...    I5-9